The invention refers to a method and a device for the true-to-original reproduction of colors and colored pictures on a screen in direct comparison to a physical color with a physical color sensor (=Softproof).
Known and customary methods and devices for comparing colors that are either available as physical models and are also reproduced on a screen, use, for example, a tube screen or an LCD screen which is mounted adjacent to an observation box (also called light box) with a surface arranged in an angle under defined illumination. An example of such a model is outlined in FIGS. 1a and 1b. An observation box 1 sits next to a monitor or screen 2. In box 1 a light source consisting of several fluorescent tubes LI-L3 is positioned diagonally across an observation surface 4. These light sources L1-L3 sends illumination or light 3 consisting of directly entering light rays 3.1 and indirect light through reflecting light rays 3.2, 3.3 onto an observation surface 4. An original color sensor 4.1 is located on the observation surface 4 in box 1, its color reproduction 4.2 appears on screen 2. Such color-matching locations are generally known and correspond to products on the market by various suppliers.
An original color sensor 4.1 to be compared will be measured for an electronic reproduction with a colorimeter or is taken with a color scanner for an entire color picture, and its color values will be reproduced on a screen as image in electronic form.
Color values as measure of colors are standardized in technology through the CIE1931 2-degree standard observer or the CIE1964 10-degree observer. The measuring technique to measure color values is described in DIN 5033 or for the graphics industry in Standard ISO 13655. Normal colorimeters supply color values either as standardized CIE1931 XYZ values (integral colorimeter), or they measure the spectral reflectance β(A.) of the color sensor (spectrophotometer). In the latter case, and with their help and with the spectral value curves x(A.), y(X) and z(X) defined by the standard observer, the standard color value X, Y and Z can be calculated:
                              X          =                      k            ⁢                                          ∫                                  380                  ⁢                  n                  ⁢                                                                          ⁢                  m                                                  780                  ⁢                                                                          ⁢                  n                  ⁢                                                                          ⁢                  m                                            ⁢                                                S                  λ                                ⁢                                  β                  ⁡                                      (                    λ                    )                                                  ⁢                                  x                  ⁡                                      (                    λ                    )                                                  ⁢                                  ⅆ                  λ                                                                    ,                  Y          =                      k            ⁢                                          ∫                                  380                  ⁢                  n                  ⁢                                                                          ⁢                  m                                                  780                  ⁢                  n                  ⁢                                                                          ⁢                  m                                            ⁢                                                S                  λ                                ⁢                                  β                  ⁡                                      (                    λ                    )                                                  ⁢                                  y                  ⁡                                      (                    λ                    )                                                  ⁢                                  ⅆ                  λ                                                                    ,                                  ⁢                  Z          =                      k            ⁢                                          ∫                                  380                  ⁢                  n                  ⁢                                                                          ⁢                  m                                                  780                  ⁢                  n                  ⁢                                                                          ⁢                  m                                            ⁢                                                S                  λ                                ⁢                                  β                  ⁡                                      (                    λ                    )                                                  ⁢                                  z                  ⁡                                      (                    λ                    )                                                  ⁢                                                      ⅆ                    λ                                    .                                                                                        G        ⁢                                  ⁢        l            
The color values are always defined as relative variable and k is a standard constant. For a precise calculation of the color values of a color sensor, it is necessary to work with the spectral distributions of the reflection of the sensor and the light. That requires that the spectral distribution S\ of the rays impinging the sensor through illumination 3 in the box is known. Therefore, this must be measured very precisely.
The fractions of the light reflected by the sensor form the so-called color stimulus and are described by the product through spectral distribution of the illumination of the sensor and the reflection of the sensor and are called spectral color stimulus, {acute over (φ)}()=Sβ(). The spectral color stimulus reflected from the sensor will be taken in by the eye of an observer and impinged on the retina of the eye where it triggers the color perception of the observer. To ascertain the standard color values, all spectral fractions are integrated through the wavelength of 380 through 780 nm visible to the human eye. The value k in G1.1 is a standardizing constant which is usually determined through the Y value of a white color sensor under the same lighting as the lightest color directly from the color value Y of the light source:
                              k          =          1                ,                              0            /                                          ∫                                  380                  ⁢                  n                  ⁢                                                                          ⁢                  m                                                  780                  ⁢                  n                  ⁢                                                                          ⁢                  m                                            ⁢                                                S                  λ                                ⁢                                                      β                                          wei                      ⁢                                                                                          ⁢                      β                                                        ⁡                                      (                    λ                    )                                                  ⁢                                  y                  ⁡                                      (                    λ                    )                                                  ⁢                                  ⅆ                  λ                                ⁢                                                                  ⁢                oder                ⁢                                                                  ⁢                ¿                                              =                                    ∫                              380                ⁢                n                ⁢                                                                  ⁢                m                                            780                ⁢                n                ⁢                                                                  ⁢                m                                      ⁢                                          S                λ                            ⁢                              y                ⁡                                  (                  λ                  )                                            ⁢                              ⅆ                λ                                                                        Gl        .                                  ⁢        2            
The color values are related variables and differ from absolute variables which are described for example in light flow or light intensity and determine the absolute brightness of a color.
For the reproduction of a color on screen 2 the starting point is the color value according to Gl. 1. The screen must be calibrated for a precise color reproduction, so that an entered color value from the technical point of view of measuring can be reproduced with the same color values on the screen. For that purpose the methods known today of the so-called “Color Managements” are being applied. The method for the reproduction of the color value on a screen has been standardized by the “International Color Consortium” (ICC) and can be viewed on the internet.
If the measured and color values reproduced under illumination in the observation box correspond metrologically with the colors that are reproduced on the screen, then a visual conformity of the two colors are expected for an observer.
So far in practice, a look-alike reproduction of an original, that is an original physical color sensor, has been only inadequately achieved. According to the Inventor's experience, there are several reasons that are listed hereinafter.
1.1 Distribution of Spectral Lighting Depending on Location
One of the first reasons for oftentimes not conforming colors is that the colorimeters used to determine the spectral reflection of a color use a specific lighting geometry as also defined in DIN 5033. This may be also a lighting directed under 45 degrees (so-called 45/0 geometry) when an observer looks perpendicular onto the sensor. Also a 0/45 Grad geometry is used, when the sensor is perpendicularly illuminated and the observer looks at the model in a 45 degree angle or a diffuse light measure with an Ulbrichtkugel (integrating sphere). A defined lighting geometry as in a colorimeter cannot be precisely reproduced in the normal light boxes. In most instances, a light source is being realized through several fluorescent tubes in the upper region of a box, in order to illuminate a sensor. To increase the homogeneity of the illumination, boxes are also known that use another fluorescent tube in the lower region of the box.
The light impinging directly from the light source on the sensor should also possess a constant spectral distribution on the observer surface, because taking into consideration a spectral distribution dependent on location when calculating the color would be very expensive. However, as a result of the reflection from the walls of the box, additionally reflected light (3.2, 3.3 in FIG. 1 b) is directed onto the sensor, which is converted through the spectral reflection properties of the walls into another spectral dispersion than the one of the direct ray 3.1. This superimposes with locally different dispersion on the direct light, e.g. near the walls, with stronger influence than in the center of the picture. Moreover, oftentimes also reflectors R1-R3 are mounted behind the fluorescent tubes, in order to guide additional light on the sensor. This measure also leads to locally different spectral fractions through the wavelength-dependent reflections of the reflectors. The illumination 3 of the sensor is thus neither fully spectrally homogenous nor has there been a reproduction of a clearly defined lighting geometry as in a measuring device.
In order to improve the distribution of brightness in the sensor area, scatter plates P are placed in front of the light sources. They are made of plastic with specific corrugation. As a result of the wavelength-dependent light refraction on plastic, an angle-dependent spectral distribution of the light-rays allowed to penetrate can be anticipated.
1.2. Effect of Interior Lighting
When color matching in practice, it is desirable to set up the observation box and the monitor in a dark room. For industrial use, usually an interior lighting is provided that makes it possible to work next to the color matching devices. However, in that instance, interior light falls also into the observation box and illuminates also the monitor surface positioned next to it. This changes in the box the perception of the original colors. In order to keep the external light fraction small, the observation box is oftentimes set up with a very large depth. The disadvantage thereof is that as a result a “cave-like” observation impression is created.
The light reflected additionally through the reflection of the screen surface because of the internal light produces a reduction of the picture contrast. Moreover, it must be noted that the spectral composition of the interior lighting is not taken into account thus far to calculate the reproducing color.
1.3 Lacking Uniformity of Illumination
Because the areas of the supporting surface of the color sensors in the observation box, in particular in perpendicular direction, are located at varying distances from the light source, the illumination strength on the supporting surface changes according to the location. On the other hand, the distribution of the luminance on a screen is not totally constant and changes according to location. Typically, differences in illumination in the observation box can occur in the range from 10% to 20%. Also with respect to screens, the typical differences of luminance from the center of the screen to the screen frame range in this area, however, their distribution differs from the distribution in the observation box. As already mentioned under 1.1., light scatter plates P are installed in the light path to improve the uniformity and moreover, an observation box is offered which uses an additional light source in the floor area to reduce the decline of the luminance in perpendicular direction. Various companies offer procedures for screens, in order to homogenize the luminance across the screen. These method effect a reduction of the difference. However, practice has shown that there are still visible differences in brightness between the colors of the originals in the observation box and the reproduction on the screen, because it can't be accomplished to exactly conform luminance distribution of the screen to the illuminance.
1.4 Environment and Color Appearance
Another problem, why in most instances no identical appearing colors can be accomplished, results from the effects of the color appearance. The visual perception of a color by a human observer depends on the distribution of colors in the direct environment of an observed sensor and on the illumination in the broader environment of a scene. Hence, the perception of brightness of a color changes drastically with the brightness of the environment and the perception of the hue shifts with the color of the environment into the direction of the complementary colors. These effects of the so-called color appearance are expressly described, e.g. in [1,2]. The shifts in the perception of color in the environment can somehow be described with the help of models. One of these models for example had been prepared and described in one working group of the International Commission on Illumination, CIE, in report CIE-159-2004. The so-called ocular adaptation level is important to the appearance of a color. Parameters that control the visual appearance of a color in the brain of a human depend on the brightness and the distribution of the colors in the field of vision and the visual system of the human gets used to it within about 2 minutes and adapts to same.
The effects of the color perception in the color matching arrangement of FIGS. 1a and 1b now effect that the colors in the original model appear differently under the illumination of the observer box in the field of view of an observer than what occurs after a shift of the observer direction on the screen with a different environment. The state of adaptation of the eye changes, if the observer looks into one direction for a longer period of time. Because the totality of all colors and brightness of the illumination in the field of vision of the observer during looking back and forth in comparison normally do not correspond, the colors on the monitor next to the box appear differently than the original colors in the observer box. Usually, when setting up the monitor, at least an environmental brightness is provided which corresponds to about 20% of the brightness of the maximum white level in the reproduction of the picture. However, in the observer box the direct environment of a color model is illuminated with the same brightness as the sensor itself.
In using a color appearance model such as known for example from CIE159-2004, an attempt can be made to compensate these differences through a predistortion of the electronically reproduced colors, however, practical experience shows that the methods available are not sufficient to achieve a precise conformity of all colors through refraction. In addition, the illumination of the environment and of the background locally fluctuates strongly and a measuring technique can only detect same with difficulty and, thus, they cannot be precisely taken into account.
1.5 Precision of Color Comparison
It must be further noted that because of the spatially separated arrangement of original and reproduced colors, a comparison of conformity is only approximately possible, because during a fast change in observation of original and reproduction, the color comparison is only approximately possible through the power of color memory of the observer. In practice, color differences during this type of comparison can only be recognized up to differences of CIE AE2000=2 to 4 (CIE AE2000 is the measure for color differences recommended by CIE). This precision is not sufficient for many applications.
Today the goal it to transmit worldwide color pictures and color sensors electronically and for professional applications and to work on design methods on the monitor. A comparison of the results of such processes is only possible, if it has been assured that on each respective monitor an original color sensor can be precisely defined, that is, up to color differences of less than ΔE2000<1.0. Even color differences of less than ΔE2000=1 are then still clearly visible, if they adjoin, which can occur frequently in practice during joining of color surfaces, e.g. then, when partial products from different product lines shall be joined together.
1.6 Observer Properties
A color comparison of original and reproduced colors oftentimes leads also to different impressions, when the spectral sensitivity curve of the type of the retinal cones of the human observer systemically deviates from those defined by standard observes according to CIE. In practice, the differences between various observers and the defined standard observer are more the rule. Moreover, the spectral color-matching curve effective for the color vision of a human eye changes with increasing age. Because the density of the retinal cone also decreases at different strength from the center [fovea-Translator] of the highest density, the color signals transmitted to the brain change with covered color surface. They are described in technology by the so-called observation angle, under which a color surface is reproduced in the eye. For that purpose 2° and 10° were selected for the standard observer.
For technical color reproduction a decision must be made for one of these two observers. For large-area color sensors the 10°-observer is used, for small-area color surfaces, the 2°-observer is used. This does not allow an exact accounting of all possible observer angles.
In principle, an adaptation for each individual observer is possible, even in dependence on age. However, a reproduction can then only be optimized for that one observer.
Patent application DE 101 21 984.9, “Methods and Devices for visually optimized representation of color pictures on monitors and/or their comparison with originals or printed pictures” is a first step disclosed, to solve the above-mentioned problems. The recommendation is outlined in FIG. 2. This recommendation suggests the use of one single box 1, which combines the observation surface 4 of the original and the monitor 2. It is recommended hereby, to arrange the observation surface 4 for one original color sensor 4.1 together with the surface of a monitor 2.2 and the color reproduction 4.2 reproduced thereon on one level and to make the colored environment for both alike. Thus, on observation level 4, an observation area 2.1 identical with the monitor surface is defined. It is also recommended to arrange a white frame 4.3 around the observation sensors or the monitor and to create around it a neutral grey environment surface. The entire surface will be illuminated as much as possible with one or several rod-shaped light sources L1. The light 3 strikes at almost 45 degrees onto the observation and monitor surface and it is assumed that an observer looks almost perpendicularly at the color mode and the monitor, that is, at an angle of α=45 degrees to the illumination. Thus, two of the above mentioned problem items under 1.1 and 1.4 are solved: The geometry of the illumination is designed as almost 45/0 degree geometry and the original color sensor and the reproduced color on the monitor are arranged in the same environment under same illumination and thus result in the same color appearance.
In patent application DE 101 21 984.9 it is also recommended to adapt the brightness of a white color, a reference white, exactly to the brightness of each original color sensor and the reproduction under joined illumination. The white frame around the observation surface and around the screen serves to stabilize the adaption state of the eye.